Consolidation of metal powder



April 13, 1954 R, D, HALL 2,675,310 n CONSOLIDATION OF METAL POWDER Filed June 23. 1949 E. D. HHLL. BY www ATTORNEY Patented Apr. 13, 1954 CONSOLIDATION OF METAL POWDER,

Roy D. Hall, Pottersville, N. J., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application June 23, 1949, Serial No. 100,798

18 Claims.

This application is a continuation-in-part'of my application Serial No. 580,429, iled March 1, 1945, similarly entitled, but now abandoned.

This invention relates to the consolidation of metal powder, and more particularly to an improvement in the method of forming metal into desired shapes in a convenient and economical manner.

The principal object of my invention, generally considered, is an improvement in the -consolidation of metal powder to form dense coherent metal and, more particularly, in connection with the more common refractory metals of series 4, 6 and 10 of groups IV, V and VI of the periodic table, that is, those of melting points at least as high as 1S00 C., forming the -group consisting of tungsten, molybdenum, tantalum, titanium, andzirconium, excluding the rare element columbium.

Another object of my invention is to form desired shapes of coherent metal by a method of powder metallurgy in which a pre-sintering or baking step, formerly employed and thought necessary in connection with the manufacture of such metals as above enumerated, is eliminated, and the iinal sintering step to nearly the temperature of fusion is made unnecessary,

A further object of my invention is the consolidation of metal powder to form dense coherent metal by pressing to consolidate the material enough to handle and heating the pressed shape in a mixture of oxygen-free gases between which and the material there is a reversible reaction because of the formation f compounds, such as chlorides, for example, at relatively-low temperas tures, said heating being to and at a temperature at which such reaction occurs.

A still further object of my invention is the method of manufacturing metal shapes, and particularly of such difcultly workable materials as above enumerated, especially those of such high melting points that casting of large ingots thereof is difficult or impossible, involving the mere pressing to the desired form and then heating to a much lower temperature than previously though necessary and in a mixture of oxygenfree gases which react between one another with respect to said metal in a reversible manner at the temperature in question. By large ingots, I mean those large enough to permit the formation of sheets, rods, and forgedv articles suitable for industrial uses.

Another object of my invention is to 'greatly' accelerate, at any temperature, the rate of sintering of pressed coherent metallic masses by heating in an oxygen-free atmosphere that reacts reversibly with the metal or metals used and thereby induces more rapid crystallization than would occur in an inert atmosphere as the result of temperature alone.

Other objects and advantages of the invention relating to the particular features, will become apparent as the description proceeds.

Referring to the drawing illustrating my invention, the single gure is an elevational view, partly in section, of a furnace which may be used in the practice of my invention.

In powder metallurgy, and particularly in the manufacture of tungsten wire, rods, sheets and the like, it has previously been thought necessary to handle as follows. Take the metal powder after reduction from the oxide, place in a mold, and press to say 15 to 30 tons per sq. inch to hold the particles together sufficiently to permit handling. When so pressed the ingot formed is initially very fragile and it has previously been the custom to subject it to a baking or prelim-'- inary sintering during which some consolidation and strengthening takes place. Such treatment makes the ingot strong enough t0 permit handling and to attach clips to the ends for the final sintering. An assembly for such a final sintering is illustrated in Fig. 1 of the Iredell et al. Patent No. 2,215,645, dated September 24, 1940. In such nal sintering, the material, if tungsten. is heated to approximately 3100 C., that is, using `current of from to 95% of that necessary to cause fusion, where the particles become so plastic that they coalesce and form material of the desired coherence.

In Patent No. 2,431,690, dated December 2, 1947, and entitled Consolidation vof Metal Powder, I and another jointly disclosed a method of sintering molybdenum and Vsimilar material, either with or without preliminary baking and shaping, in a furnace containing a hydrogenbearing reducing gas impregnated or saturated with water vapor. By means of treatment in this reactive atmosphere for a length of time much longer than that of the conventional method described, we were able to lower the sintering temperatures required for such metals as `molybdenum and tungsten, as there described. The sintering temperature suitable for molybdenum was found to be about 1600 C., or between 1500"v C. and 1700 C., and is within the range of temperatures obtainable in a molybdenum-wound resistance furnace, such as is commonly available.

However, for tungsten, higher temperatures are required than can be readily obtained by presently available furnaces, usually necessitating for that metal induction heating for long periods of time, such as two or three hours, which is not as simple or satisfactory as the employment of a resistance furnace would be. It is therefore desirable to 4nd asinteri'ng. atmosphere which is more reactive when such metals as molybdenum and tungsten are being treated,

as well as to find one in which there is a re:-`

versible reaction for sinteringsuch, metals...as tantalum, titanium and zirconium, since Vthe oxides of these three are not reducedcbyihydrogen, and the metals tl'iems'elveszare.oxidzized'byV water vapor at temperatures above about"500f'C.

In accordance with my invention,.I.prQpose to use a halogen atmosphere, or one consisting of or including chlorine or other halogen,.-except fiuorine which is not convenient to use, as the reactive gas. or vapor. .Chlorine combines with tungsten at relatively .low temperatures, .that is, under 600 C., in accordance .with .the'equatiom W-i-BClzL-VVCls. At higherstemperatures, that is, about 2000 C., tungstenchloride. decomposes into metallic tungsten and chlorine. Atintermediate temperatures, that is, between 10.00 C. and 1500 C., and particularly at low pressures, either partial or total, there is eiective afreversible reaction, combining the formation and decomposition of tungsten chloride .at the same temperature. A similar reaction occurs with molybdenum. Thisv phenomenon offers a method of sintering bars, of any of the five metals mentioned, made from pressed powders, into ingots of high density and purity,wcapable of being worked or forged to any .desired shape or use.

In accordance with-the above, I propose to press a bar of tungsten powder, for example, in a manner similar to that now used in making tungsten wire in accordancev withthe conventional method. Thisbar or slug is then transferred to a furnace and heated to the desired temperature about but not lower than 1200 C., and below 1500 C., in an atmosphere comprising or formed by a member of the halogen group of chlorine, bromine and iodine-at `low pressure, as one alternative, to sinter said powder intoa bar for working. Ano-ther alternative is to use as an atmosphere, in which to effect such sintering, 'some inert gas such-.as nitrogen, .helium or argon, or even hydrogen, but'preferably argon or 'helium at atmospheric pressure; .the selected gas being in such case mixed with -a small percentage of any one of thefselected halogens, above mentioned, preferably chlorine.

If irregular or special shapes are required, these can be obtained by pressing the.powder to the desired shape prior `to firing,- or bak-.ing for fa. relatively short period of. time, such as twenty minutes, at a mild temperaturein a protective or reactive atmosphere, such as one of the mentioned gases, halogenated as described, and shaping, while allowing for shrinkage, prior to sintering in accordance with the alternative method of the Hall et al. patent referred to. Although chlorine is preferred because of its cheapness, one of the other halogens, say Ibromine or iodine, may be employed. The. partial pressure of the halogen is desirably between 50 and 100,microns (that is, between about .0066% and .01.32% by volume) and thusthe amount need not exceed 1% of the mixture If .an undiluted. halogen atmosphere is employed, its pressure shouldbe low, desirably from about 50 microns to not more than about 10 millimeters of mercury.

The reactions of molybdenum and tungsten with the halogens being Very similar, it is possible to apply this process to molybdenum at temperatures even lower than those used for tungsten. However, the process of the Hall et al. patent, previously. referred to, is Yvery lsatisfactory for molybdenum,.and simpler to practicethan the present process.

If one of the other metals mentioned, such as tantalum, titanium or zirconium is to be sintered, the diluting atmosphere when employed is limited to. inertA gas, such as helium or argon, since :both nitrogenfand-hydrogen combine with such metals valthough for the purpose considered inert with respect. to tungsten and molybdenum. In accordance with my invention, I propose a. iinal; sintering treatment at a very much lower temperature than formerly thought necessary, making .theprocess operable by incorporating a halogen, preferably chlorine, in vthe treating gas and employing a, treating temperaturelcorresponding with that at which a reversible. reaction. between the metal and the components of the treating gas occurs. 'The use of a gas or vapor bringing about a reversible reaction between the associated elements, gives a method of cementing or coalescing bodies to a dense mass approximately equivalent in density and structure to what would occur if the metal were actually melted and cast in a mold, the action, however, occurring at practical speeds at'temperatures far below the melting point of metal, and only about. 25% .to 50% of the melting temperature as measured in degrees C., that is, at temperatures not lower than 25% and not higher than 50% of the melting temperatureof the selected metal.

-I willillustrate this .specifically for tungsten but a similar process is applicable to `molybdenum, tantalum, titanium, and zirconium in either a mixture of an inert` gas, such as helium or argon, and a halogen, such as chlorine, or such a halogen at low pressure.

By reversible reaction I means one which takes place in both 'directions at a selected temperature. For example W-i-BCl'zeWCls. In other words, the associated materials are inthe state of flux, the tungsten 'in this case, for example, being subject to. chlorination, and the tungsten chloride being, in turn, subject to decomposition. Molybdenum, zirconium, tantalum, and titanium react in the presence of chlorine giving reversible reactions of the same character, only varying in range of temperature.

A similar reaction for tungsten can be carried out in gas media .involving other halogens and halogenacid gases, such .as hydrogen chloride, as follows: YW-l-HCkWCls-I-SHzl As an'l explanation. of how the. reaction occurs, itis suggested that an atomic phase, or sort of vapor phase, of the metal exists. at the moment of these chemical reactions, so that the atoms are by forces of crystallization drawn toward crystal nuclei and incorporated therewith. These forces are great enough to cause shrinkage of the apparently rigid body at temperatures well below the melting point, giving. mechanical properties approximating those obtained by actual fusion of the powder and casting to shape.

As a specific example of what has-been accomplished by my invention, a tungsten ingot formed bypressing .the-powderunder standard conditions maybe firedV innitrogencontaining under 1%:.of

chlorine, by volume, in a furnace at a temperature lower than 1500 C., say about 120 C., for about two hours. When removed, it will have shrunk 15% to 20%, say from one inch to 63 inch in section, and be fully treated. It will forge readily, being in all respects equivalent to an ingot treated in the conventional manner. I therefore accomplish, by heating to a temperature well below what was considered to be the necessary final sintering temperature, what has previously been accomplished by both a preliminary sintering and a final sintering to nearly the temperature of fusion, the reductions in sintering temperature being such that, even when treating metal as difficult to work as tungsten, it is possible to eifect the sintering operation in a molybdenum-wound resistance furnace or the equivalent.

Referring now to the drawing in detail, there is shown in the sole figure, ingots or slugs id, which may have been formed in a press comprising a die or mold with a removable bottom piece, in which mold is placed a desired quantity of the selected metal, for example, tungsten, in finely powdered form. Said powder is then pressed into ingots or slugs by the plunger, so that they may be removed without breakage and satisfactorily handled.

Although I am not limited to any special type of furnace, the slugs i4 produced may be passed through a treating furnace such as that designated it in the figure. Thus furnace may be of the general type illustrated in Figure l of the Hall et al. Patent No. 2,431,690 previously referred to. That is, it may comprise a suitably insulated tube Il which, in this instance, is formed of Pyrex, Vycon as defined in the Rentschler Patent No. 2,469,410, dated May l0, 1949, porcelain, Royal Berlin porcelain, quartz or high-tem perature-resistant glass so as to not only resist the temperature of treatment, but also the action of the halogen gas or vapor at a relatively high sintering temperature. The tube il desirably extends continuously from the front or free end of the preheating chamber portion i 8 of the furnace, through the sintering chamber i3, and on to the extreme end of the contiguous cooling chamber 2 i That portion of the tube il which defines the sintering chamber it is wound with a resistance or high-frequency coil 22 of suitable material, such as molybdenum or tungsten wire, through which electrical energy is passed to provide the desired heating effect. II'he coil 22 is in turn surrounded by alumina or other suitable insulation 23, preferably of the powdered type for effectiveness, and the whole enclosed in apreferably airtight metal casing 25 and supported on a suitable table or bench 2t. However, an air-tight casing is not necessary if the furnace tube il is impervious to the gases used therein. Ify tungsten or molybdenum is to be treated, the treating mixture may be nitrogen impregnated as described with chlorine, bromine, or iodine, helium, argon or hydrogen similarly impregnated, or the selected halogen without other admixed gas.

If, on the other hand, we are instead treating tantalum, titanium or zirconium, I use a gas such as helium or argon, instead of nitrogen or hydrogen, impregnated with chlorine or other selected halogen gas or vapor, because such rare carrying gases will not react with such metals to make brittle compounds. Whatever mixture is selected, it is desirably introduced at the preheating end of the furnace, as through valve 223 and pipe 2l through cork of inlet door 3l, and preferably withdrawn from a pipe 29 through cork of outlet door 32 at the other end of the furnace. The cooling chamber portion 2l desirably has a suitable water jacket 33 provided with a water inlet pipe 34 and an outlet pipe 35.

The slugs I4 of pressed powder are desirably held in suitable boats 36, formed of porcelain or other material inert to the treating gas, and introduced upon removal of the cork, or through the door, 3|, into the preheating compartment from whence they are gradually pushed, manually or by means of a pusher element 3l, corresponding with that designated 25 in the Hall et al. patent, previously referred to, into the sintering compartment I9. Said compartment is at such time held at the desired temperature, say about 1200 C., or between 1G00" C. and l500 C., and the tungsten slugs maintained therein, say for about two hours, from one and one half to three hours, or until the particles thereof have sintered as desired, whereupon they are pushed on into the cooling chamber 2l for eventual removal through the outlet opening upon removal of the cork, or opening of the door, 32. The corresponding temperatures for treating molybdenum are about 950 C., or between 750 and ll50 C.

Although inlet and outlet doors, corresponding with the hinged door 22 of the Hall et al. patent previously referred to, may be employed at the inlet and outlet ends of the furnace, corks or other air-tight closures are in this case preferred to prevent leakage of poisonous gas or vapor. Where, however, the furnace atmosphere is carried at a pressure well below atmospheric, the continuous process becomes complicated and the furnace charge is then preferably carried through its treating cycle in a closed furnace, that is, one wherein the treating gas may be conveniently kept at the desired low pressure.

In the manufacture of complicated shapes, in order to avoid subsequent grinding or machining the die, containing article or articles to be pressed enclosed in a rubber casing, may be immersed in water or other liquid in a closed container andv hydrostatic pressure applied thereto, whereby the pressure is uniform over the entire mold area, as disclosed in the Hall et al. patent previously refer-red to. Although this method does not adapt itself to large automatic production, especially of small articles, yet for such purpose, molds could be used with multiple cavities to give large output. The dies or molds need not be made to withstand from one side only the strain of the high pressures, since in use the pressure is uniform from all sides.

In accordance with one embodiment of my invention, I take metal powder, selected from the group of metals mentioned, and place it in a die to make a blank for the finished article desired. The powder may be pressed, as by a plunger, so that upon removal it can be satisfactorily handled.

Blanks so produced may then be passed through a treating furnace, such as illustrated in the drawing, wherein they are heated to a relatively mild temperature while protected against oxidation by an atmosphere, such as disclosed in conmay be; further decreased, vas disclosed in the 4I-Iall etal. patent previously reierred to. 1n some instances Vit may be heated at temperatures so low that the articles produced have not much more than just sufficient strength to withstand the contemplated shaping operation.

This initial temperature can be determined by notingat what temperature shrinkage begins and not `allowing the temperature to rise aboveA such temperature causing shrinkage. In the caseof molybdenum, this in hydrogen is from 1000" C. to 1200" C. when the treating period is from 10 to 20 minutes, or about 1160" C. when the treating period is about an hour. 1n the case of tungsten, temperatures higher than i300" C. as from 1300" to i490" C. are employed for from 10 to 20 minutes. Temperatures in the low part of this range require longer periods of time. In halogenated atmospheres, the temperatures are correspondingly lower.

After the pressed blanks have been moved through the furnace, as by means of an endless belt or chain passing over pulleys, not shown, or by a pusher element 25, as described in the Rennie Patent No. 2,402,034, and have moved through or beenheld in any desired manner for a desired period in such a preliminary firing furnace, the same are removed therefrom and subjected to desired .machining for similar working operations.

The .machined or otherwise worked blanks are then passed through the same or a similar' furnace and heated in a gas of the group consisting of nitrogen, inert gas, and hydrogen mined with matter o the group consisting of chlorine, bromine andiodine to a tei iperature, not higher than 50% of that fusion as measured in degrees C., and not lower than 25% of that of fusion, similarly measured, to a temperature at which a reversible reaction betveen such materials occurs, to cause the particles to sinter together to the desired strength. This is, in the case of tungsten, a temperature of about i200" C. or between 1000" and 1500" C., and in the case of molybdenum, a temperature of about 950 C. or between 750" and 1150" C. It wil be seen that this sintering temperature, forming the last step of the embodiment just described, corresponds with the sintering describedin connection with the rst embodiment.

Such production of the molded articles and nnishing in nnal form by merely heat treatment;

in a furnace at relatively low temperature, either with or without preliminary heating and working, is believed to be a radical advance in the art of powder metallurgy. The making of such shapes by the old method, involving rlrst baking or preliminary sint ring and then nal sintering between electrode., where the ends aregripped and the temperature of the sin'tering is nearly as highY as the melting point of the metal, was not possible because said method is only applicable to ingots of uniform cross section and where it .is permissible to discard the ends of the ingot, gripped in the clamps of the sintering bottle, as not being fully treated.

However, in accordance with my invention all parts of the ingot are treated to the same extent. Therefore, in pressing the article into shape there is no waste of untreated end portions and it is only necessary to make aliowance for the shrinkage when the treating occurs. This, of course, is very simple and is well understood to those skilled in the art in connection with casting metal where a similarallowance is made.

The pro-cess of my invention can also be used to `cement or weld the same or metalsother than thosermentioned to each other, provided they respond toathe :sameV vreversible gaseor -vapor reaction, within a common range vof temperature and vapor pressure. Thus two standardl ingots can be united'endto end by heating to the sintering temperature while in close contact, 1000o C. to 11500" C., in the case-of tungsten, in chlorinated or halogenatedfgaseasuch as nitrogen, hydrogen, helium-and argon, until the two have coalesced or practically welded together, thereby decreasing the number of` pieces to be handled for swaging and drawing. Metal bodies with non-metallic elements or metallic compounds incorporated thereinto may also be produced in accordance with my invention. Inall of these instances, an undiluted vhalogen at low pressure may be substituted for halogenated gas.

It is understood that even higher temperatures than thoseA indicated can be used in all cases, the time of sintering being greatly` reduced b y the useof the above described reversible reaction.

The range of temperature for tungsten has been reduced so low that furnaces of conventional design can be used, permitting the preparation of nishedshapeswhch previously could not be treated, .but had to be fashioned fromv rod or bar. lvIy process increases greatlythe sise of the piece, as well as the variety of shapes, thatcan be formed. instead of a treating bottle limited to rods and barsofuniform. cross-section and within the current-carrying capacity of the contacts, I can now make any shape andsize that can be contained in afurnace which can `be treated to 1400" C., and lledwith an atmosphere of hydrogen or other .gas mentioned mixed with a very low percentage of chlorine by volume. As an example, slabsof tungsten vfor rolling can be pressed to any desired width, avoiding the necessity of cross rolling.

My idea takesadvantagepf the rapid ,crystallisation induced by the halogen-halide or chlorine-chloride vcycle to accelerate sintering orcementing the metal particles into adense workable form. This can be carried on atany tempera-- ture abovethe minimurrnbut not at any temperature where this reaction takes place, as I have toraise the temperature sufficiently high so that this effect israpid enough torelease large forces which pull the particles together. 1200" C. for tungsten.

However, increaseV in temperature increases this force and decreases thetime required for,

the maximum effect. Also, the greater the pressure used for pressing, the closer the metal particles are drawn together andthe easier to get good workable metal attheselow temperatures.

It has, however, not beenfound necessary to in-` crease the pressure over that normally used, to

obtain as dense an ingot as those obtained:

formerly.

./.lthoughA preferred. embodiments of our ir vention have been disclosed, ,itwill be understoodpressed shape irreu flowingstreamof an inerty gas mixed with matter of therg-ltoup consisting of chlorine, bromine.. 4and iodine,- to, aftcmperature- This is about.

9 not higher than 50% of that of fusion of the metal to be consolidated, as measured in degrees C., at which a reversible reaction between such materials occurs, whereby said powder is coalesced to a mass approximately equivalent in density and structure to what would be obtained if actually melted and cast in a mold.

2. The method of consolidating metal powder of the group consisting of tungsten, molybdenum, tantalum, titanium, and zirconium to form dense coherent metal, comprising pressing to shape the powder, and heating the pressed shape for a relatively long period of time in a owing stream of an inert gas mixed with matter vof the group consisting of chlorine, bromine, and iodine, to a temperature not higher than 50% of that of vfusion of the metal to be consolidated, as measured in degrees C., at which a reversible reaction between said mixture and the selected metal occurs.

3. The method of consolidating metal powder of the group including tungsten, molybdenum, tantalum, titanium, and zirconium to form dense coherent metal, comprising pressing to consolidate the powder, and heating the pressed shape in a flowing stream of gas in which the active portion is only matter of the group consisting of chlorine, bromine, and iodine, at a pressure between 50 microns and 10 millimeters of mercury to a temperature not higher than 50% of that of fusion of the metal to be consolidated, as measured in degrees C., at which a reversible reaction between suoli materials occurs.

4. The method of consolidating tungsten powder to form dense coherent metal, comprising pressing to shape the powder, and heating the pressed shape in a flowing stream of an inert gas .mixed with matter of the group consisting of chlorine, bromine, and iodine, to a temperature not higher than 50% of that of fusion of tungsten, as measured in degrees C., at which a reversible reaction between such materials occurs.

5. The method of consolidating tungsten powder to form dense coherent metal, comprising 1 pressing to shape the powder, and heating the pressed shape in a flowing stream of an atmosphere of an inert gas admixed with matter of the group consisting of chlorine, bromine, and iodine, to a temperature between 1000 C. and 1500 C.

6. The method of consolidating tungsten powder to form dense coherent metal, lcomprising pressing to shape the powder, and heating the pressed shape, in a flowing stream of a mixture of chlorine and nitrogen to a temperature of about 1200 C. for about two hours.

7. The method of consolidating tungsten powder to form dense coherent metal, comprising pressing to shape the powder, and heating the pressed shape in a flowing stream of chlorinebearing hydrogen to a temperature not higher than 50% of that of fusion of tungsten, as measured in degrees C., at which a reversible reaction between the associated materials occurs.

8. The method of consolidating tungsten powder to form dense coherent metal, comprising pressing to shape the powder, and heating the pressed shape in chlorine-bearing hydrogen to a temperature between 1000 C. and 1500 C.

9. The method of consolidating tungsten powder to form dense coherent metal, comprising pressing to shape the powder, and heating the pressed shape in chlorine-bearing hydrogen to a temperature of about 1200 C. for about two hours.

10. The method of yconsolidating tungsten powder to form dense coherent metal, comprising pressing to shape the powder, and heating the pressed shape in a flowing stream of nitrogen at atmospheric pressure and containing from about .01% to 1% of chlorine by volume to a temperature of about 1200 C. for about two hours.

11. The method of consolidating tungsten powder to form dense coherent metal, comprising pressing to shape the powder, and heatingthe pressed shape in a ilowing stream of chlorine at a pressure between about 50 microns and 10 millimeters of mercury to a temperature not higher than 50% of that of fusion of tungsten, as measured in degrees C., at which a reversible reaction between such materials occurs,

12. The method of consolidating molybdenum powder to form dense coherent metal,` comprising pressing to shape the powder, and heating the pressed shape in a flowing stream of an inert gas mixed with matter of the group consisting of chlorine, bromine, and iodine, to a temperature not higher than 50% of that of fusion of molybdenum, as measured in degrees C., at which a reversible reaction between such materials occurs.

13. The method of consolidating molybdenum powder to form dense coherent metal, comprising pressing to shape the powder, and heating the pressed shape in a flowing stream of an inert gas admixed with mattei' of the group consisting of chlorine, bromine, and iodine, to a temperature between 750 C. and 1150 C.

14. The method or" consolidating molybdenum powder to form dense coherent metal, comprising pressing to shape the powder, and heating the pressed shape, in a flowing stream of a mixture of chlorine and nitrogen to a temperature of about 950 C. for about two hours.

15. The method of consolidating molybdenum powder to form dense coherent metal, comprising pressing to shape the powder, and heating the pressed shape in a owing stream of nitrogen at atmospheric pressure and containing from about .01% to 1% of chlorine by volume to a temperature of about 950 C. for about two hours.

16. The method of consolidating molybdenum powder to form dense coherent metal, comprising pressing to shape theI powder, and heating the pressed shape in a owing stream of chlorine at a pressure between about 50 microns and 10 millimeters of mercury to a temperature not higher than 50% of that of fusion of molybdenum, as measured in degrees C., at which a reversible reaction between such materials occurs.

17. The method of making articles from metal powder of the group consisting of tungsten, molybdenum, tantalum, titanium, and zirconium comprising pressing to shape the powder, heating the pressed powder in a protective atmosphere at a mild temperature high enough to give it merely sufficient strength to withstand working, shaping the article to finished size, while allowing for subsequent sintering shrinkage, and iinally heating said article in a flowing stream of an inert gas mixed with matter of the group consisting of chlorine, bromine, and iodine, to a temperature not higher than 50% of that of fusion of the metal being heated, as measured in degrees C.. at which a reversible reaction between such materials occurs, until the particles are sintered to dense coherent metal of the desired strength.

18. The method of making articles from tungsten powder, comprising pressing to shape the powder, heating the pressed powder in an inert gas miXed with matter of the group consisting of chlorine, bromine, and iodine, at a relatively low temperature, just long enough to merely give it sufficient strength to withstand working, shaping the article to finished size, while allowing for subsequent sintering shrinkage, and nally heating said article in a flowing stream of a similar halogenated inert gas to a temperature between 1000 and 1500 C. for about two hours, until the particles of tungsten are sintered to dense coherent metal of the desired strength.

References Cited in the file 0f this patent UNITED STATES PATENTS Number Name Date v 912,246 Kuzel Feb. 9, 19`09 1,074,333 Thowless Sept. 30, 19,13 1,123,625 Thowless Jan. 5, 1915 Number Number Name Date Oberlander Dec. 12, 1916 Thowless May.22, 1917 Laise July 18, `1922 Bradford May 1, 1923 'Williams et al Sept. 13, 1927 Kroll June 25, 1940 Harrison July 3, 1951 .FOREIGN PATENTS Country Date Germany Oct. 29, 1936 Great Britain Dec. 27, 1940 OTHER `REFERENCES Jones: Principles .ofPowder Metallurgy, published by Edward Arnold andCo., London 1937,

page.60. 

1. THE METHOD OF CONSOLIDATING METAL POWDER OF THE GROUP CONSISTING OF TUNGSTEN, MOLYBDENUM, TANTALUM, TITANIUM, AND ZIRCONIUM, COMPRISING PRESSING TO SHAPE THE POWDER, AND HEATING THE PRESSED SHAPE IN A FLOWING STREAM OF AN INERT GAS MIXED WITH MATTER OF THE GROUP CONSISTING OF CHLORINE, BROMINE, AND IODINE, TO A TEMPERATURE NOT HIGHER THAN 50% OF THAT OF FUSION OF THE METAL TO BE CONSOLIDATED, AS MEASURED IN DEGREES C., AT WHICH A REVERSIBLE REACTION BETWEEN SUCH MATERIALS OCCURS, WHEREBY SAID POWDER IS COALESCED TO A MASS APPROXIMATELY EQUIVALENT IN DENSITY AND STRUCTURE TO WHAT WOULD BE OBTAINED IF ACTUALLY MELTED AND CAST IN A MOLD. 